Oldest Evidence for Dinosaurs in Tiny Footprints

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The earliest known fossils associated with dinosaurs have been
identified in 250-million-year-old rocks from Poland.

The fossils -- footprints made by dinosaur relatives known as
dinosauromorphs -- suggest that dinosaurs evolved from small,
four-legged animals that lived during the Early Triassic just a
few million years after the "Great Dying," Earth's most severe
extinction event to date.

"For some reason, the major dinosaur lineages survived this
extinction -- we don't know exactly why, and it may have been
little more than random fortune -- and they probably then had the
freedom to flower in a post-apocalyptic world," lead author
Stephen Brusatte told Discovery News.

"(Dinosauromorphs) are the very closest relatives to dinosaurs,
animals that were right on the cusp of becoming dinosaurs, shared
many features with dinosaurs, probably looked and behaved like
dinosaurs, but are not bona fide dinosaurs by definition,"
Brusatte, a paleontologist at the American Museum of Natural
History, explained.

The finding was published in the Proceedings of the Royal
Society B.

Brusatte and colleagues Grzegorz Niedzwiedzki and Richard Butler
analyzed multiple fossilized animal tracks dating from the Early
and Middle Triassic at the Holy Cross Mountains in southern
Poland, an exciting new frontier for early dinosaur research.

The oldest tracks from this site are only about a half an inch in
length, so the scientists conclude they belonged to an animal
that was approximately the same size as a modern housecat,
weighing at most around four pounds. Its hind legs were also
longer than its forelimbs, since the footprints overstep the
handprints.

The best way to identify a track-maker is to tie features of the
footprint with those known to be present in the skeleton of the
suspected animal's foot. Often this is a difficult process, since
many footprints only preserve vague outlines and not much other
anatomical information.

Three primary attributes of the dinosauromorph tracks allowed the
research team to connect the fossils to this group of animals.

First, he said the three central digits of the track-maker are
dominant and the outer "toes" are reduced. This arrangement, he
explained, is unique to dinosaurs and their closest relatives
versus the larger digits of other animals from the time, such as
lizards and crocodile-like archosaurs.

"Second," he continued, "the digits are essentially parallel, and
this reflects the unique condition of dinosaurs and close
relatives in which the foot is a tightly bunched structure."

The third characteristic is that the tracks reflect the unique
ankle development of dinosaurs and their closest relatives.
"(It's) a simple hinge and not a more complex rotary joint like
the ankles of crocodile-line archosaurs, lizards and even
humans," he said.

Martin Ezcurra of the Bernardino Rivadavia Natural Sciences
Museum told Discovery News that he agrees with the study.

"This discovery forces us to think on an older evolutionary
history for this group and, for the first time, in a probable
relation with the biggest massive extinction in the history of
the Earth, the Permo-Triassic extinction around 250 million years
ago," Ezcurra said.

Michael Benton, professor of vertebrate paleontology at the
University of Bristol, thinks the conclusions of the study are
"undeniable," including the new theory that dinosaurs and their
close relatives date to the Early Triassic rather than the Late
Triassic.

"The Earth had been devastated by massive climactic changes at
the end of the Permian, and pulses of flash warming continued
through the Early Triassic, some five million years, continually
destabilizing the environment and delaying full recovery of
ecosystems," Benton said.

"This was the rather grim world the first dinosaurs may have
seen," he added.